This invention relates in general to communication systems, and particularly, to a method for transmitting collision-free messages in a communication system and apparatus therefor.
Presently, there are many communication medium access protocols in existence for exchanging messages between decentralized communication devices utilizing a common channel. Ethernet is an example of a commonly used communication protocol in decentralized local area networks. Aloha is another.
Ethernet networks employ a channel acquisition technique generally known as CSMA/CD, which stands for Carrier Sense Multiple Access Collision Detection. Essentially, this technique requires a communication device (e.g., a personal computer) to sense a carrier signal of another communication device actively using the communication medium prior to acquiring the channel. If activity is detected, the communication device postpones acquisition of the channel according to a random delay. If no activity is detected, the communication device begins transmitting a message immediately. If upon transmitting the message a collision is detected, the communication device terminates transmission, and postpones acquisition of the channel according to a random delay.
In Aloha networks, on the other hand, a communication device acquires a channel and begins transmission of a message without sensing for other users. A collision is detected upon the communication device failing to receive an acknowledgment signal from the recipient communication device within a specified time. Upon detecting a collision, the communication device postpones reacquisition of the channel according to a random delay. It should be apparent that the potential for collisions in the decentralized communication systems described above makes their use inefficient under high traffic loads. This is especially a problem in systems where a short latency is desirable. Centralized communication systems also may insert unnecessary latency into a lightly loaded system. Accordingly, a method and apparatus is needed for minimizing message collisions without degrading channel utilization efficiency.